Junction high electron mobility transistor-heterojunction bipolar transistor (JHEMT-HBT) monolithic microwave integrated circuit (MMIC) and single growth method of fabrication

ABSTRACT

A highly uniform, planar and high speed JHEMT-HBT MMIC is fabricated using a single growth process. A multi-layer structure including a composite emitter-channel layer, a base-gate layer and a collector layer is grown on a substrate. The composite emitter-channel layer includes a sub-emitter/channel layer that reduces the access resistance to the HBT&#39;s emitter and the JHEMT&#39;s channel, thereby improving the HBT&#39;s high frequency performance and increasing the JHEMT&#39;s current gain. The multi-layer structure is then patterned and metallized to form an HBT collector contact, planar HBT base and JHEMT gate contacts, and planar HBT emitter and JHEMT source and drain contacts.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to mixed-signal device fabrication technology andmore specifically to a JHEMT-HBT MMIC and method of fabrication thatrequires only a single growth process.

2. Description of the Related Art

Mixed-signal devices take advantage of the different characteristics offield effect transistors and bipolar transistors to achieve circuitfunctions and performance levels not currently available using existingsingle-device technologies. HEMTs have very low noise and high currentgain characteristics which make them well suited for detecting faintsignals. However, HEMTS are highly non-linear, and thus cannot operateover a large dynamic range. HBTs are relatively noisy but highly linear,and thus well suited for the amplification of large signals. As aresult, the two complementary devices can be combined to form a lownoise amplifier (LNA), with the HEMT providing the front end receiverand the HBT providing a high linearity output stage. Furthermore, knownanalog or digital bipolar circuit configurations can be used to controla HEMT amplifier. For example, an HBT circuit can be used to regulatethe current in the HEMT amplifier.

One known approach is to fabricate separate HEMT and HBT MMICs thatperform the respective amplification and biasing functions, bond them toa carrier and then interconnect their pins using conventional wirebonding techniques to produce the desired mixed-signal circuitconfiguration. The advantage of this approach is that the fabrication ofthe single-device MMICs is well known. However, the mixed-signal circuitis not an integrated circuit, and thus does not have the cost andperformance advantages of an IC. Wire bonding separate MMICs is laborintensive and thus expensive, requires large device-to-device spacingwhich reduces the device density per wafer, and adds resistance to thecircuit which has the effect of lowering its speed.

Streit et al, "Monolithic HEMT-HBT Integration by Selective MBE," IEEETransactions on Electron Devices, Vol 42, No. 4, pp. 618-623, April,1995 discloses a selective growth process for integrating aSchottky-gate HEMT and an HBT in a MMIC. First, a Gallium-Arsenide(GaAs) substrate is placed in a chamber and a multi-layer npn-HBTstructure is grown on the substrate using a molecular beam epitaxy (MBE)process. The substrate is removed from the chamber and patterned toroughly define the HBT device. The patterning process includesdepositing a layer of silicon nitride on the HBT structure, depositing amask over the silicon nitride and plasma etching the exposed siliconnitride with chlorine-fluoride gas to selectively remove the siliconnitride and define the HBT device. The silicon nitride patterningprocess is expensive and time consuming.

The substrate is then put back into the chamber for a second MBEregrowth to produce the multi-layer HEMT structure, followed by etchingthe substrate with hydrofluoric acid to remove the remaining siliconnitride from the HBT structure. The compositions of the multi-layer HBTand HEMT structures are selected to optimize their respectiveperformances. Thus, the materials and thickness of the individual layersare not the same. Once both the HBT and HEMT structures are formed, theyare patterned and metallized using conventional etching and depositionprocesses, respectively, to define the devices active areas and metalcontacts. This includes a gate-recess etch through the HEMT structuresto define their Schottky barriers. The gate-recess etch is difficult tocontrol, and thus reduces the HEMT uniformity across the wafer.

Although Streit's HEMT-HBT is integrated, and thus realizes theadvantages of integrated circuits, it has a number of seriousdeficiencies. The growth and regrowth of the HBT and HEMT structuresincreases fabrication time which increases the cost of the MMIC. Whenthe substrate is removed from the chamber between growths for processingit can become contaminated, which reduces the quality of the HEMTmaterial grown in the regrowth stage. Furthermore, during regrowth theHEMT material tends to build up along the edge adjacent the HBTstructure. As a result, the periphery of the HEMT can be of poor qualityand unusable as part of the HEMT's active area. This increases thespacing between the devices, which reduces the number of devices thatcan be fabricated on a wafer. The regrowth process also exposes thedevice to temperatures in excess of 600° C., which causes dopants in thehighly doped base region to diffuse into the emitter and collectorregions, thereby reducing the abruptness of the pn junctions andlowering the HBT's current gain.

Streit's growth and regrowth process produces HEMT and HBT devices thatare non-planar, i.e., they have large step discontinuities betweenadjacent devices. This occurs because the two devices are fabricated inindependent growths, and thus cannot be exactly matched, have differentmulti-layer structures to optimize their respective performances, andthe HEMT structure builds up near the edge of the HBT during regrowth.As a result, the potential for breaks in the metal interconnectionsformed through conventional deposition processes is high. This reducesthe reliability of the HEMT-HBT device. Furthermore, thicker metalinterconnections are required to reduce the chance of breakage due tothe step discontinuities, which increases the weight of the device.

Zampardi et al, "Circuit Demonstrations in a GaAs BiFET Technology,"Solid-State Electronics, Vol. 38, No. 9, pp. 1723-1726, 1995 disclose aSchottky-gate MESFET-HBT integrated circuit. The MESFET and HBT shareonly a single layer; the MESFET's channel and the HBT's emitter. Inorder to provide the necessary HBT emitter characteristics, Zampardiuses GaAs, which provides lower channel mobility than that achievable ina HEMT.

Usagawa et al., "A New Two-Dimensional Electron Gas Base Transistor(2DEG-HBT)," IEDM, pp. 78-81, 1987 discloses an integrated pnp-HBT HEMTdevice in which the devices share a single layer; the HEMT's channel andthe pnp-HBT's base. The shared layer includes a two dimensional electrongas (2DEG) that is necessary to provide the high electron mobility inthe HEMT. The 2DEG is very thin, and thus the HBT's base is susceptibleto punch through when heavily reverse biased. This can destroy the HBT'sbipolar operation. Furthermore, the pnp-HBT is very slow due to the lowmobility of holes with respect to that of electrons.

SUMMARY OF THE INVENTION

In view of the above problems, the present invention provides a singlegrowth process for fabricating a highly uniform, planar and high speedJHEMT-HBT MMIC.

This is accomplished by growing a multi-layer semiconductor structure ina single growth such as an MBE process. The multi-layer structureincludes a composite channel-emitter layer, a base-gate layer and acollector layer. The channel-emitter layer comprises an emitter/channellayer in which the HBT's emitter region and the JHEMT's channel areformed, a sub-channel/emitter layer on one side of the emitter/channellayer that is doped to reduce the access resistance to the emitterregion and the channel, and a composite modulation layer on the otherside of the emitter/channel layer that is doped to form a heterojunctionat the JHEMT's channel.

The multi-layer structure is patterned and metallized to form acollector pedestal that supports an HBT metal collector contact.Thereafter, the base-gate layer is patterned to a) form an HBT base mesathat supports an HBT metal base contact adjacent to the HBT metalcollector contact and which extends below the collector pedestal, and tob) form a JHEMT gate pedestal which supports a JHEMT metal gate contactthat is spaced apart from the HBT collector contact. An HBT emittercontact is formed on the composite channel-emitter layer adjacent theHBT base contact, and the JHEMT source and drain contacts are formed onthe composite channel-emitter layer on opposite sides of the JHEMT gatecontact. The HBT emitter and JHEMT source and drain contacts areactivated to infuse them into the composite channel-emitter layer suchthat the HBT emitter contact defines the emitter region and the JHEMTsource and drain contacts define the channel.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a single growth fabrication of a JHEMT-HBT MMICin accordance with the present invention;

FIG. 2 is a perspective view of a preferred JHEMT-HBT MMIC produced bythe single growth fabrication;

FIGS. 3a through 3g illustrate successive steps in the fabricationprocess as seen along section line 3--3 in FIG. 2;

FIG. 4 is a band gap energy diagram for a JHEMT produced by thefabrication process shown in FIGS. 3a through 3g;

FIG. 5 is a band gap energy diagram for a HBT produced by thefabrication process shown in FIGS. 3a through 3g;

FIG. 6 is a sectional view of an alternative inverted JHEMT-HBT MMIC;and

FIG. 7 is a band gap energy diagram for the inverted JHEMT shown in FIG.6.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a single growth process for fabricatinga JHEMT-HBT MMIC, in which the JHEMT and HBT devices share two layers;the JHEMT gate/HBT base layer and the JHEMT channel/HBT emitter layer.This improves the planarity of the mixed-signal MMIC, which increasesthe reliability of the metal interconnects formed by overlay deposition.Furthermore, the uniformity between devices on a wafer depends only uponthe growth uniformity and not on the subsequent patterning. The MMICfeatures a collector up HBT structure which exhibits a lowcollector-base capacitance and thus has a higher operating frequencythan known emitter up HBTs. It also includes a composite emitter-channelstructure that provides low emitter resistance and low channel accessresistance required for high frequency operation.

Another aspect of the invention is that the multi-layer structure isdesigned so that the performance of one device is not traded off againstthe performance of the other. Their functions in a given layer areconsistent, and thus selecting a material or a dopant level to improvecertain properties in one device is consistent with improving the otherdevice. For example, the selection of the material for the barrier layerprovides a large valence band barrier that reduces hole injection intothe JHEMT's channel and improves the HBT's emitter efficiency.

As illustrated in FIG. 1, the JHEMT-HBT MMIC is fabricated by placing asubstrate in a growth chamber (step 10) and, in a single growth, formingthe entire multi-layer JHEMT-HBT structure on the substrate (step 12).The multi-layer structure includes a composite channel-emitter layer, abase-gate layer and a collector layer. The structure also preferablyincludes a buffer layer for smoothing the surface of the substrate, anetch stop and channel cap layer for facilitating the selectiveundercutting of the HBT's base and reducing the access resistance to theHBT's emitter contact and the JHEMT's source and drain contacts, and acontact layer for reducing the access resistance between the collectorand its metal contact. An MBE growth process is typically used toprovide the best growth uniformity. However, MBE provides a relativelyslow growth and is therefore expensive. Depending upon the application,it may be more efficient to use deposition processes such as metalorganic chemical vapor deposition (MOCVD) or an organic metal vaporphase epitaxy (OMVPE), which are faster but less uniform. The singlegrowth process avoids the contamination and the high temperaturereexposure that are associated with a re-growth and degrade theperformance of known MMICs.

Once the multi-layer structure is grown, the substrate is removed fromthe growth chamber and patterned/metallized to define the JHEMT's andHBT's active areas and metal contacts (step 14). As a result, the JHEMTand HBT are spaced closely together, with a gap of approximately 10 μmbetween them, without edge degradation. Furthermore, the HBT's emitterand JHEMT's source and drain contacts are replanar and the HBT's baseand JHEMT's gate contact are replanar. This has the effect of improvingthe reliability of an overlay metal deposition (step 16) which followthe pattern/metallization. Thereafter, topside vias and interconnectmetal deposition are formed using conventional techniques (step 18).

A portion of a JHEMT-HBT MMIC 20 showing a single pair of JHEMT and HBTdevices 22 and 24, respectively, fabricated in accordance with thesingle growth process described in general in FIG. 1 and in more detailin FIGS. 3a through 3g, is shown in FIG. 2. The MMIC 20 includes asubstrate 26 and preferably a buffer layer 28 for smoothing the surfaceof the substrate. A composite channel-emitter layer 30 on the bufferlayer 28 serves as the active channel and emitter areas for the JHEMTand HBT, respectively. An etch stop layer 32 on the compositechannel-emitter layer 30 provides low access resistance to the HBT'semitter contact 34 and the JHEMT's source and drain contacts 36 and 38,respectively. The emitter contact 34 and the source and drain contacts36 and 38 are interfused into the composite channel-emitter layer 30 todefine an emitter region 40 and a channel 42, respectively.

A patterned channel cap layer 44 on the etch stop layer 32 supports anHBT base mesa 46 above the emitter region 40 and also a JHEMT gatepedestal 48 above the channel 42, which respectively support an HBT basecontact 50 and a JHEMT gate contact 51. The JHEMT's pn junction pedestalgate provides better dc and rf uniformity than the well known Schottkygate HEMT. The channel cap layer 44 is undercut to cut off the path forfree electrons from the emitter region 40 to the HBT base contact 50 toimprove emitter injection efficiency. Alternately, the channel cap andetch stop layers can be omitted and the path between the base contact 50and the emitter region 40 cut off by ion implantation of the portion ofthe base mesa 46 below the base contact 50. A collector pedestal 52 onthe base mesa 46 supports an HBT collector contact 54. A low resistancecontact cap 56 preferably separates the collector pedestal 52 andcontact 54 to reduce the access resistance to the contact collector 54.

The emitter region 40, base mesa 46 and collector pedestal 52 togetherdefine the collector up vertical HBT 24, in which under forward bias ofits base-emitter junction, high mobility electrons are injected from theemitter region 40, drift through the base, and are collected at thecollector electrode 54 due to the field established by the collectorvoltage. The channel 42 and gate pedestal 48 define the horizontal JHEMT22, in which electrons are swept through the channel from the sourcecontact 36 to the drain contact 38 in response to a gate voltage appliedto the gate contact 51 that modulates the channel 42. The two devicesare electrically isolated and physically separated by approximately 10 mby an implant isolation 57.

FIGS. 3a through 3g illustrate the fabrication of a preferrednon-inverted n-channel JHEMT npn-HBT MMIC 20. As shown in FIG. 6 theMMIC can also be fabricated with an inverted JHEMT, which lowers theturn-on voltage of the HBT. The MMIC could also be fabricated as ap-channel JHEMT pnp-HBT device. However, this is not currently preferredbecause the mobility of holes is substantially lower than that ofelectrons.

As shown in FIG. 3a, an undoped insulating GaAs substrate 26 having athickness of 400-600 μm is placed in an MBE chamber 58 under anultrahigh (deep) vacuum. The substrate 26 is heated and bombarded by ionbeams 60 produced by gallium (Ga), arsenide (As), indium (In) andphosphide (P) ion sources 62 to form a high-quality semiconductor filmon the substrate. Closing or opening shutters 64 controls the filmcomposition and/or doping levels to within one atomic distance.

Because the surface of the substrate 26 is relatively rough, the bufferlayer 28 is grown to provide a smooth surface as shown in FIG. 3b. Thebuffer layer is suitably an undoped large band gap material with highresistance and is grown to a thickness of 1 Å to 1 μm. Thereafter, thecomposite channel-emitter layer 30 is grown on the buffer layer 28.First, an n-type GaAs sub-emitter/channel layer 66 is grown to athickness of 50-500 Å with a dopant level of 1×10¹⁷ to 5×10¹⁸ impurityatoms/cm³. The sub-emitter/channel layer 66 provides low channelresistance, less than 1000Ω per square, a high electron concentrationfor the JHEMT and a low emitter access resistance for the HBT. Second,an intrinsic InGaAs emitter-channel layer 68 is grown to a thickness of50-500 Å to provide high electron mobility, a low band gap voltage(0.35-1.1 ev), and low effective electron mass. Third, a compositemodulation layer 70 is grown on the emitter-channel layer 68.

The composite modulation layer 70 includes an intrinsic GaInP spacerlayer 72 on the emitter-channel layer 68 with a thickness of 10-100 Å,an n-type GaInP donor layer 74, and an intrinsic to slightly n-type(greater than 5×10¹⁷ atoms/cm³) GaInP barrier layer 76. (When aninverted JHEMT-HBT is fabricated, the sequence of growth steps for thecomposite emitter-channel layer 30 is reversed so that the barrier layeris grown on the substrate and the sub-emitter-channel layer is on top.)

The donor layer 74 is grown with a sheet thickness of 1 Å to 100 Å andis typically specified by the product of its thickness and the number ofdonor atoms/cm³, suitably between 1×10¹² and 3×10¹² atoms/cm². The donorlayer 74 supplies high mobility free electrons to the emitter-channellayer 68, leaving ionized impurity atoms in the donor layer. The spacerlayer 72 isolates the free electrons in the channel from their ionizedimpurity atoms, thereby reducing ionized impurity scattering. Thebarrier layer 76 has a relatively high band gap voltage (1.1-2.5 ev)that creates a heterojunction 78 at the interface between the compositemodulation layer 70 and the emitter-channel layer 68. The material anddopant levels of the barrier layer are selected to provide both a largeconduction band discontinuity that constrains the free electrons to thechannel, and a large valence band barrier that limits hole injectioninto the channel. The composition of the barrier layer 76 is alsoconsistent with providing the HBT 24 with a high emitter efficiency.

The n-type GaAs etch stop layer 32 is preferably grown on the barrierlayer 76 to a thickness of 40-100 Å with a dopant level between 5×10¹⁷and 5×10¹⁸ atoms/cm³. The etch stop layer 32 provides low accessresistance to the emitter, source and drain contacts shown in FIG. 2 andfacilitates selective removal of the channel cap layer 44. The channelcap layer is suitably 40-100 Å of n-type GaInP with a dopant level of5×10¹⁷ and 5×10¹⁸ atoms/cm³ and may be selectively undercut as shown inFIG. 2 to improve emitter injection efficiency.

A p⁺ -type GaAs base-gate layer 80 is grown on the channel cap with ahigh dopant level of 1×10¹⁹ to 1×10²¹ to provide the desiredbase-to-emitter properties and a low access resistance to the base andgate contacts 50 and 51 shown in FIG. 2. The base-gate layer ispreferably as thin as possible (200-1000 Å) to increase speed, yet thickenough to prevent punch through. An n-type GaInP collector layer 82having a thickness 50 Å to 1 μm with dopant levels of 5×10¹⁷ and 5×10¹⁹atoms/cm³ is grown on the base-gate layer 80. The collector layer isselected so that it can be removed from the base-gate layer and canwithstand high electric fields. Alternately, the collector layer can beformed with a relatively thick GaAs layer and a thin GaInP thatfacilitates removal from the base-gate layer 80. Lastly, a 100 Å to 1 μmn⁺ -type contact layer 84 with a dopant level in excess of 5×10¹⁸atoms/cm³ is preferably formed on the collector layer 82 to reduce theaccess resistance to the collector contact 54 shown in FIG. 2.

Once the JHEMT-HBT structure has been grown, the substrate is removedfrom the chamber and patterned/metallized to define the devices' activeareas and metal contacts shown in FIG. 2. The preferred approach is topattern the metal contacts and then use them to etch the underlyinglayer to define the active area. Alternately, conventionalphotolithographic techniques can be used to first pattern the activeareas and then deposit the metal contacts. The preferred approachrequires fewer processing steps and produces metal contacts and activeareas that are self-aligned. The alternate approach would allow thecollector and base contacts to be deposited at the same time.

In accordance with the preferred fabrication, the collector contact 54is formed on the surface of contact layer 84 as shown in FIG. 3c. Thisis preferably accomplished by forming a polymer layer over the contactlayer, exposing it to radiation and developing it to remove the polymermaterial where the collector contact is to be formed. Metal is thenevaporated over the polymer layer to form the contact 54 and the polymeris lifted off.

As shown in FIG. 3d, the collector contact 54 has been used as a mask toacid etch and selectively remove the exposed portion of the contactlayer 84 and the underlying collector layer 82. Using the same processthat was used to form the collector contact 54, the base contact 50 andgate contact 51 are formed on the base-gate layer 80 as shown in FIG.3e. Thereafter, the base-gate layer 80 is patterned to define the basemesa 46 and gate pedestal 48. As shown in FIG. 3f, the base mesa isselectively undercut by etching away the cap layer 44 to cut off thepath between the base contact 52 and the emitter region.

As shown in FIG. 3g, the HBT emitter contact 34 and JHEMT source anddrain contacts 36 and 38 are deposited on the etch stop layer 32 andactivated to interfuse them into the sub-emitter/channel layer 66. TheHBT emitter contact 34 defines the emitter region 40 and a sub-emitter86 that reduces the access resistance between the emitter region and theemitter contact, thereby increasing the maximum operating frequency ofthe HBT. The JHEMT source and drain contacts 36 and 38 define thechannel 42 and a sub-channel 88 that reduces the access resistancebetween the channel and the source and drain contacts, therebyincreasing the JHEMT's current gain. The two devices are electricallyisolated and physically separated by approximately 10 μm by an implantisolation 57.

FIG. 4 illustrates the band gap energy diagram 90 for the horizontalJHEMT under equilibrium conditions. The energy diagram shows thediscontinuities in both the conduction band E_(c) and valence band E_(v)at the heterojunction 78.

The donor layer 74 supplies free electrons that produce a 2DEG in thechannel 42. The spacer layer 72 separates the free electrons in the 2DEGfrom ionized impurity atoms that remain in the donor layer 74. Thebarrier layer 76 is selected and doped so that the hole barrierpotential φ_(p) is greater than the electron barrier potential φ_(n)and, as a result, the hole injection into the channel 42 is low. In thegate pedestal 48 the valence band energy E_(v) is slightly above thefermi energy E_(f) and the conduction band energy E_(c) is approximately1.1-2.5 ev higher. The JHEMT's channel 42 is modulated by the potentialapplied at the gate contact 51. This regulates the flow of the freeelectrons between the source and drain contacts 36 and 38, respectively,along a horizontal axis into the plane of the paper.

FIG. 5 illustrates the band gap energy diagram 92 for the verticalnpn-HBT under equilibrium. The pn base-emitter junction of the HBTshares the same requirements as the pn gate-channel junction of theJHEMT shown in FIG. 4. The conduction and valence bands E_(c) and E_(v)of the n-type collector pedestal 52 and emitter region 40 are lower thanthose of the p⁺ -type base pedestal 52. Because the electron barrierpotential φ_(n) is greater than the hole barrier potential φ_(p), theHBT has high emitter injection efficiency. Under forward bias, the HBTis controlled by modulating the base potential to regulate theconductivity between the collector pedestal 52 and emitter region 40 toproduce a voltage gain across their contacts.

FIG. 6 is a sectional view of an inverted JHEMT-HBT 94. The inverteddevice is formed using the same single growth fabrication discussed inrelation to FIGS. 3a through 3g except that the sequence of growing thecomposite emitter-channel layer is reversed. As a result, the barrier76, donor 74, and spacer 72 layers are grown between the buffer layer 28and the emitter-channel layer 68, and the sub-emitter/channel layer 66is grown on top of the emitter-channel layer 68.

As shown in FIG. 7, the energy band characteristics for the invertedJHEMT-HBT are flipped around the heterojunction 78 in the band gapenergy diagram 96. This reduces the HBT's base-emitter turn-on voltageby moving the energy spike 98 at the heterojunction outside the HBTemitter region 40. The performance of the inverted JHEMT is onlyslightly degraded when compared to the non-inverted JHEMT.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. For example, an InP substrate can be usedinstead of a GaAs substrate. The multi-layer structure would be the sameexcept that the GaInP materials are replaced with InP and the GaAsmaterials are replaced with GaInAs. Other combinations of materials anddopant levels may be used in the single growth fabrication process toproduce the JHEMT-HBT MMIC. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

We claim:
 1. A Junction High Electron Mobility Transistor-HeterojunctionBipolar Transistor (JHEMT-HBT) monolithic microwave integrated circuit(MMIC), comprising:a substrate; an emitter-channel layer over saidsubstrate; an implant through said emitter-channel layer that definesand isolates coplanar HBT emitter region and JHEMT channel; coplanaremitter contact and source and drain contacts, said emitter contactinterfused into said emitter region and said source and drain contactsinterfused into said channel; a composite modulation layer configuredand positioned to form a heterojunction with said emitter-channel layerand to supply high mobility free electrons to said channel; a base-gatelayer positioned above said emitter-channel layer, said base-gate layerpatterned to form coplanar HBT base mesa and JHEMT gate pedestal thatare respectively arranged over said emitter region and said channel;coplanar base and gate contacts on a first portion of said base mesa andon said gate pedestal, respectively; an HBT collector pedestal on asecond portion of said base mesa; and a collector contact on saidcollector pedestal, said HBT emitter region, base mesa and collectorpedestal forming a collector up vertical HBT and said JHEMT channel andgate pedestal forming a horizontal JHEMT.
 2. The JHEMT-HBT MMIC of claim1, further including a sub-channel/emitter layer abutting saidemitter-channel layer and doped to reduce the channel resistance and theaccess resistance to said emitter region.
 3. The JHEMT-HBT MMIC of claim2, further comprisinga contact cap on the collector pedestal thatreduces access resistance to said collector contact.
 4. The JHEMT-HBTMMIC of claim 1, wherein said composite modulation layer is between saidemitter-channel layer and said base-gate layer.
 5. The JHEMT-HBT MMIC ofclaim 1, wherein said composite modulation layer is between saidsubstrate and said emitter-channel layer.
 6. A Junction High ElectronMobility Transistor-Heterojunction Bipolar Transistor (JHEMT-HBT)monolithic microwave integrated circuit(MMIC), comprising:a substrate; acomposite channel-emitter layer on said substrate, said channel-emitterlayer patterned to have an HBT emitter region and a JHEMT channel; a HBTemitter contact and source and drain contacts on and interfused intosaid composite channel-emitter layer respectively adjacent said emitterregion and on opposite sides of said channel; a base-gate layerpositioned above said composite channel-emitter layer, said base-gatelayer patterned to have an HBT base mesa over said emitter region and aJHEMT gate pedestal over said channel; base and gate contacts on a firstportion of said base mesa and on said gate pedestal, respectively; anHBT collector pedestal on a second portion of said base mesa; acollector contact on said collector pedestal; wherein said compositechannel-emitter layer comprises: an emitter-channel layer having a lowband gap energy, said JHEMT source and drain contacts defining saidchannel in said emitter-channel layer; an n-type sub-channel/emitterlayer on one side of said emitter-channel layer, saidsub-channel/emitter layer being doped to provide a low channelresistance and to reduce resistance between said emitter region and saidHBT emitter contact and between said channel and said JHEMT source anddrain contacts; and a composite modulation layer on the other side ofsaid emitter-channel layer, said composite modulation layer supplyinghigh mobility free electrons to said channel and spacing said freeelectrons away from the ionized donor atoms to reduce ionized impurityscattering, and having a high band gap energy that provides both aconduction band discontinuity and a valence band barrier at saidchannel; said HBT emitter region, base mesa and collector pedestalforming a collector up vertical HBT and said JHEMT channel and gatepedestal forming a horizontal JHEMT.
 7. The JHEMT-HBT MMIC of claim 6,wherein said emitter-channel layer is between said substrate and saidcomposite modulation layer.
 8. The JHEMT-HBT MMIC of claim 6, whereinsaid composite modulation layer is between said substrate and saidemitter-channel layer.
 9. The JHEMT-HBT MMIC of claim 6, wherein saidemitter-channel layer has conduction and valence band energies that setits band gap energy, said composite modulation layer comprising:a spacerlayer on said emitter-channel layer to space the free electrons from theionized donor atoms to reduce ionized impurity scattering in saidchannel; an n-type donor layer on said spacer layer to supply the highmobility free electrons to said channel; and a barrier layer on saiddonor layer, said barrier layer having conduction and valence bandenergies that are respectively higher and lower than those of saidemitter-channel layer to provide said conduction band discontinuity andsaid valence band barrier at said channel.
 10. The JHEMT-HBT MMIC ofclaim 6, wherein said composite modulation layer includes a barrierlayer that provides a large valence band barrier to limit hole injectioninto said channel.
 11. The JHEMT-HBT MMIC of claim 6, furthercomprising:an n-type etch stop layer on said composite emitter-channellayer; and a patterned n-type channel cap layer on said etch stop layerthat supports said base mesa, said channel cap layer undercut to reducea path for free electrons from said emitter region to said base contact,to improve emitter injection efficiency.